The operation of many systems depends upon their ability to transmit signals through a particular transmission medium. If possible, a medium is preferably selected to conduct transmissions without interference or disruption, although some attenuation typically occurs. Often, a medium of this type is relatively confined and uniform.
In most instances, however, a system is required to transmit signals through a medium that may be disruptive. For example, radar and communication systems regularly transmit electromagnetic energy through the atmosphere. Although the beam of such transmissions may be controlled, to some extent, by the system, the atmosphere is an unconfined, multipath transmission medium that can affect the quality of the transmitted signal.
In a multipath medium, a transmission may follow several different paths as it travels from one point to another. Because of differences in the various paths, a portion of a transmission traveling along one path may reach a given point at a different time than a portion of the transmission following another path. Similarly, path differences may cause the phase of a portion traveling along one path to differ from the phase of a portion traveling along another path.
As will be appreciated, the receipt of various portions of a transmission at different times and with different phases can make interpretation of the transmission difficult. To address this problem, systems have been developed that, for example, eliminate potentially conflicting portions of a transmission upon receipt. One such system is described in the Background section of U.S. Pat. No. 4,669,091.
The system disclosed in the above-referenced patent uses a plurality of tapped delay lines to simulate the delays experienced by transmissions traveling along various multipaths. The tapped delay lines generate delayed replicas of the portions of the signal received from the various paths. These replicas are summed to produce an approximation of the distortion components in the received signal. This approximation is then subtracted from the received signal to produce an equalized signal, largely free of multipath distortion attributable to path length variations.
The delays required to produce the appropriate replicas can be manually adjusted for fixed channels or transmission media. If the transmission medium is in a state of flux, as in the case of an airborne radar system for detecting other airborne targets, however, manual control is typically inadequate to achieve the frequent adjustments required. As a result, some form of automatic control of system parameters is typically required.
In that regard, one approach described in this prior art patent involves the periodic transmission of a training signal having known characteristics, in place of the data. The influence of the multipath distortion upon the known training signal allows the characteristics of the distortion to be evaluated. With the multipath distortion characterized, the delays required to allow replicas of the distortion components to be produced can be easily determined. Unfortunately, the use of such a training signal increases system complexity and, because it must be periodically transmitted in place of data, reduces the system's data transmission capacity.
The referenced patent itself discloses an alternative approach to the control of a multipath distortion correction equalizer for use with data burst communications in which no knowledge of the data content of the signal is available. In that regard, the multipath delay is established by autocorrelation and the phase shift and amplitude of a multipath replica, representing the multipath delayed components, are adjusted to minimize the noncarder spectral components of the replica. A received signal is then corrected by subtracting from it the multipath replica.
As will be appreciated, the approaches described above may be successfully used to remove or cancel multipath distortion from a received signal. In many applications, however, it would be desirable to retain and constructively use the multipath components. This approach is especially important in radar systems where the multipath components can be separated by time delay. More particularly, by summing the various multipath components received, it may be possible to increase the overall signal strength and, hence, the system's range. The useful summation of multipath distortion components can, however, be difficult to achieve, particularly in airborne applications, due to the varied and unpredictable nature of the multipath echo.
As the multipath environment varies, the multipath components received by the system will also vary. In some instances, these components may additively enhance the received signal, while, in other instances, the multipath components may destructively cancel the received signal.
On average, the coherent addition of the multipath components to the received signal increases the power of the received signal (because ##EQU1## In spite of this average increase, however, the destructive influence of the multipath components (for certain values of relative phase, .theta.) may be so great that it renders the received signal undetectable. As a result, even though the average performance of the system is enhanced, numerous "dropout" or "no-coverage" zones may exist.
In view of these observations, it would be desirable to provide a multipath system that processes multipath distortion components so as to increase the strength and power of received signals. It would further be desirable to provide such a system that ensures continuous coverage even when the multipath environment varies. Finally, it would be desirable for such a system to be relatively uncomplicated.